A 62-year-old man ended up in the emergency room with hand tremors and heart palpitations six months after he began taking the weight-loss drug tirzepatide, also known as Zepbound. According to his doctors, the man’s heart predicament was likely caused by an overdose of his thyroid medication that wasn’t adjusted properly following his rapid weight loss from tirzepatide.
Tirzepatide is the latest incretin medication, a class of drugs that mimics gut and brain hormones important to regulating hunger and metabolism. It’s the first approved incretin to combine two of these hormones at once, GLP-1 and GIP—a dual-action mechanism that makes it more potent at treating obesity and high blood sugar. In clinical trials, people have lost an average 20% or more of their baseline weight, well above the typical success seen with diet and exercise alone. These results have also surpassed the average 15% weight loss seen with Novo Nordisk’s semaglutide (sold under the names Ozempic and Wegovy), the first of this newer generation of incretin drugs. Tirzepatide is sold under the name Mounjaro to treat type 2 diabetes and as Zepbound to treat obesity. But in this particular case, published Monday in the journal JAMA Internal Medicine, the drug’s effectiveness looks to have been an double-edged sword.
According to the report, the man visited an emergency department complaining of palpitations, excessive sweating, confusion, fever, and hand tremors. Testing soon revealed that he had developed an atrial fibrillation, or an irregular heartbeat. The man had a history of type 1 diabetes, obesity, and autoimmune hypothyroidism, the latter being a condition where the immune system attacks the thyroid, which then leads to a constant shortage of essential thyroid hormones. He had long been taking insulin for his diabetes and the drug levothyroxine for his hypothyroidism, but six months before his ER visit, he was also started on tirzepatide for his weight. The man was around 290 pounds before taking tirzepatide, and he lost more than 80 pounds in half a year.
Unfortunately, the dosage of levothyroxine needed to mimic a healthy thyroid can be affected by lots of things, including body weight. And the man’s weight loss meant that he was now taking too much levothyroxine. His atrial fibrillation and resulting symptoms were officially determined to be caused by an overflow of thyroid hormones (formally called thyrotoxicosis) in “the context of rapid weight loss from tirzepatide.” Thankfully, he received immediate treatment for his condition in the emergency department.
Other research has generally found that incretin drugs may have even more health benefits than currently thought. A study this February found that tirzepatide can help keep blood pressure in check, while another study in April indicated that it could be used to treat sleep apnea. But the man’s doctors say this case should provide a “teachable moment”—the lesson being that every drug, no matter how useful, can have risks that should be proactively monitored by clinicians. For situations like this, doctors should consider checking and readjusting people’s levothyroxine dosage every 4 to 6 weeks, at least initially, they said. They also note there are other drugs affected by weight that should be reevaluated while taking incretins, such as hormonal birth control.
